Total synthesis of (+)-phenguignardic acid, a phytotoxic metabolite of Guignardia bidwellii

Article abstract of DOI:10.1002/ejoc.201300531

(+)-Phenguignardic acid, a phytotoxic metabolite of the grape black rot fungus Guignardia bidwellii was synthesized, as well as its enantiomer, in eight steps from (R)-phenyllactic acid and 3-phenylprop-2-yn-1-ol. The formation of the carboxylate at a late stage, to avoid enolization of the precursor used in the central acetalization step, proved to be crucial. The synthesis of the natural product allowed the unabiguous assignment of its hitherto unknown absolute configuration. (+)-Phenguignardic acid, a potent phytotoxic metabolite of Guignardia bidwellii, was synthesized, as well as its enantiomer, in eight steps from (R)-phenyllactic acid and 3-phenylprop-2-yn-1-ol. The absolute configuration of the natural product was determined to be S. Copyright

Full text of DOI:10.1002/ejoc.201300531

FULL PAPER
DOI: 10.1002/ejoc.201300531
Total Synthesis of (+)-Phenguignardic Acid, a Phytotoxic Metabolite of
Guignardia bidwellii
Alexander Stoye,[a] Danuta Kowalczyk,[a] and Till Opatz*[a]
Keywords: Total synthesis / Natural products / Heterocycles / Phenguignardic acid / Phytotoxins
(+)-Phenguignardic acid, a phytotoxic metabolite of the
grape black rot fungus Guignardia bidwellii was synthesized,
as well as its enantiomer, in eight steps from (R)-phenyl-
lactic acid and 3-phenylprop-2-yn-1-ol. The formation of the
carboxylate at a late stage, to avoid enolization of the precur-
sor used in the central acetalization step, proved to be cru-
cial. The synthesis of the natural product allowed the unabig-
uous assignment of its hitherto unknown absolute configura-
tion.
Introduction
Grape black rot, a devastating disease in grapes, is caused
by the fungus Guignardia bidwellii and affects vineyards in
North America[1] and continental Europe, in particular in
the Moselle and the Rhine valley.[2] Its proliferation is pro-
moted by climate change. Other Guignardia species are re-
sponsible for diseases in the horse chestnut, in citrus plants,
or in tropical fruits such as papayas.[3] The major phyto-
toxic metabolite of G. bidwellii is (S)-guignardic acid (1,
Figure 1),[4] but its congener phenguignardic acid (2)[5]
shows an even more pronounced effect. Alaguignardic acid
(3),[6] another metabolite of G. bidwellii, shows nearly the
same phytotoxic activity as 1. Compounds 1–3 presumably
originate from amino acid metabolism (Scheme 1); their
putative precursors would be the oxidative deamination
products of valine, alanine, and phenylalanine (dimethylpy-
ruvic acid, pyruvic acid, and phenylpyruvic acid), respec-
tively.[7]
Scheme 1. Phenguignardic acid, a formal dimer of phenylpyruvic
acid.
Results and Discussion
The synthesis of 1 and the determination of its absolute
configuration have already been reported by Rodrigues-
Heerklotz et al.,[4] but no synthesis of 2 has previously been
published.
Moreover, the absolute configuration of the quaternary
stereocenter of 2 remained unknown. The CD spectra of
1 and 2 are almost perfect mirror images, thus suggesting
different absolute configurations for the two compounds
(vide infra).
Here we describe the synthesis of both enantiomers of
phenguignardic acid (2) and the unambiguous assignment
of the hitherto unknown absolute configuration of the natu-
ral product. Taking account of the common synthetic strat-
egy for the formation of 1,3-dioxolan-4-ones from ketones
and α-hydroxy carboxylic acids,[4,8] we initially attempted to
establish the quaternary stereocenter by diastereoselective
acetalization with enantiopure (R)-phenyllactic acid (4,
Scheme 2) and phenylpyruvic acid benzyl ester (5) as the
starting materials. As a consequence of the strongly favored
enolization, however, 5 did not exhibit sufficient carbonyl
reactivity to react either with phenyllactic acid (4) or with
phenylpyruvic acid (to yield acetals 6a or 7, respectively).
Figure 1. Phytotoxic metabolites of Guignardia bidwellii.
[a] Institute of Organic Chemistry, University of Mainz,
Duesbergweg 10–14, 55128 Mainz, Germany
E-mail: opatz@uni-mainz.de
Homepage: http://www.chemie.uni-mainz.de/OC/AK-Opatz/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300531.
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Synthesis of a Guignardia bidwellii Metabolite
Deprotonation of 8a/8b with lithium diisopropyl amide
(LDA), treatment of the lithium enolates with phenylselenyl
bromide, and addition of H2O2/HOAc effected syn-elimi-
nation from the corresponding selenoxides (Method A),
yielding the model compounds ent-9 and 9 in 34% and
29% yields, respectively.[9] Radical bromination of 8a with
N-bromosuccinimide and 2,2Ј-azobis(2-methylpropio-
nitrile) (AIBN) in boiling CCl4 furnished the desired α,β-
unsaturated dioxolanone ent-9 in 50% yield (Method B).[10]
Radical bromination of similar 1,3-dioxolan-4-ones has
been reported to yield the corresponding 5-bromo-1,3-diox-
olan-4-ones,[4,10] but in our case dehydrobromination oc-
curred spontaneously under these conditions, so no ad-
ditional base-induced elimination was required.
The results obtained from ECD spectroscopic examina-
tion of 9 were contradictory. The ECD spectra of natural
2 and of model compound 9 were almost superimposable
(Figure 2), thus suggesting the S configuration for 2. On
the other hand, comparison of the ECD spectra of natural
Scheme 2. Phenylpyruvic acid benzyl ester (5) showed insufficient
carbonyl reactivity in ketalization reactions.
Experiments carried out at elevated temperature (benz-
ene at reflux) in combination with use of boron trifluoride
etherate afforded acetal 6b (Scheme 2), the result of an un-
desired dealkoxycarbonylation reaction, in 13% yield.
Suppression of the enolization in 5, either by hydrogen-
ation of the benzene ring or by reduction of the ester group
to the alcohol, might allow successful acetalization with α-
hydroxy acids. This was checked by choosing phenylprop-
an-2-one as a model substrate. Treatment of this with 4
yielded the 1,3-dioxolanones 8a (trans, Scheme 3) and 8b
(cis) in 72% combined yield and a 1.5:1 diastereomeric ra-
tio. Flash chromatography on silica gel furnished the pure
diastereomers 8a (41%) and 8b (27%) along with 4% of a
diastereomeric mixture. The relative configurations of the
two products were assigned on the basis of two-dimensional
NMR experiments (NOESY; see the Supporting Infor-
mation).
Figure 2. ECD spectra of natural 2 (line), of 9 and ent-9 (dashed),
and of (S)-2 and (R)-2 (dotted).
Scheme 3. Synthesis of the enantiomerically pure model compounds 9 and ent-9.
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2 and the spectra of (S)-1 reported by Heerklotz et al.[4] premature cleavage of the silyl ethers, the formation of side
(data not shown) suggested the R configuration for 2, be- products dominated.
cause all the bands were inverted (Table 1). The inversion
of the ECD bands was thus caused either by the exchange
of the ester group in 2 for the methyl group in 9 or by the
presence of the benzyl residue in 2 instead of the isopropyl
group in 1.[6,11]
Removal of chloroacetate after installation of the olefinic
double bond turned out to be problematic. A reason for
this could be undesired vinylogous additions of the S-nu-
cleophiles used for deprotection. The protecting group was
therefore removed first, by treatment either with HDTC
(hydrazine dithiocarbamate, prepared in situ from hydraz-
ine hydrate and carbon disulfide) in a mixture of acetic acid
and 2,6-lutidine,[13] or with thiourea[14] in N,N-dimethyl-
formamide. Cleavage protocols involving thiourea usually
require elevated temperatures (typically 70 °C), but depro-
tection of 12a/12b was successfully completed at room tem-
perature. Both procedures gave comparable yields of the
primary alcohols 13a/13b.
Table 1. Chiroptical properties: overview.
Entry
Compd. Absolute
configuration
[α]D Δε,
299 nm
Δε,
226/232 nm
1
2
3
4
5
1[a]
9
ent-9
2
ent-2
S
(–) (–)
(+) (+)
(–) (–)
(+) (+)
(–) (–)
(+)
(–)
(+)
(–)
(+)
R[b]
S[b]
S
R
Because of their instability in acidic media and their ten-
dency to undergo transacetalization reactions, the unpro-
tected dioxolanones 13a/13b were immediately used in the
[a] See ref.[4] [b] Note that the CIP nomenclature differs due to the
higher priority of the 2-benzyl vs. the 2-methyl substituent.
Our synthesis of 2 started with commercially available 3- next step without purification. Treatment with triethylsilyl
phenylprop-2-yn-1-ol, which was converted into 1-hydroxy- trifluoromethanesulfonate (TESOTf) and triethylamine in
3-phenylpropan-2-one (10, Scheme 4) in a two-step pro- dichloromethane gave silyl ethers 14a/14b, which were puri-
cedure.[12] After acylation of the free hydroxy group with fied by RP-HPLC, after which radical bromination/dehy-
chloroacetyl chloride and treatment with (R)-2-hydroxy-3- drobromination (vide supra) furnished (2S)-2-benzyl-5-
phenylpropanoic acid in neat BF3·OEt2 (Scheme 4), the di- benzylidene-1,3-dioxolan-4-one [(S)-15, 36%, reaction A,
oxolanones 12a and 12b were isolated in 94% yield (dr 2:1). Scheme 5]. Significant interconversion of 14a/14b, probably
Separation of the diastereomers by silica gel flash caused by sequential ring opening/closing reactions of the
chromatography furnished the desired major product 12a acetal ring, was observed, along with the formation of the
(32%) along with its epimer 12b (19%) and a mixture of TES-protected ester of phenyllactic acid 14c (Scheme 4,
both diastereomers (9%). The loss of material during sepa- 9%). No additional deprotection step was required, due to
ration is due to partial decomposition.
the concomitant cleavage of the silyl ether by the liberated
In contrast, earlier attempts involving TIPS- or TBDPS- hydrogen bromide. In the presence of 2,6-di-tert-butylpyr-
protected 10 as the ketone components in the acetalization idine, the deprotection could be effectively suppressed. In
step gave complex reaction mixtures. Although we were able this case, the reaction time had to be extended (reaction B,
to isolate the desired products, the yields were much lower Scheme 5). However, partial hydrolysis of the TES group
than in the case of chloroacetyl protection. As a result of during purification led to diminished yields of (S)-16 [23%
Scheme 4. Synthesis of the protected dioxolanones; molecular structure of the isolated byproduct 14c.
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Synthesis of a Guignardia bidwellii Metabolite
Scheme 5. Synthesis of (S)-2 and (R)-2.
(contained several impurities) from 14a] and the isolation
of a product mixture containing (R)-15 (33%) and (R)-16
(2%, both from 14b). Finally, oxidation with a combination
of catalytic amounts of 2,2,6,6-tetramethylpiperidinyl N-ox-
ide (TEMPO) and an excess of PhI(OAc)2 in acetonitrile/
water (1:1) furnished the target compounds (S)-2 and (R)-
2 in 45% and 35% yields, respectively.[15]
Experimental Section
General Methods: All reactions were carried out in dried glassware
under inert atmosphere (argon) in anhydrous solvents with use of
standard syringe and septa techniques. Anhydrous THF was dis-
tilled from potassium/benzophenone under argon. Anhydrous
dichloromethane was distilled from CaH2 under argon. The sol-
vents used for flash chromatography were distilled prior to use; the
It is noteworthy that the TES ether (S)-16 was trans- solvents used for analytical and preparative HPLC were of “gradi-
ent grade” quality and were degassed in an ultrasonic bath prior
to use. Anhydous DMF (water Յ0.005%, over molecular sieves)
was ordered from a commercial supplier and used as received.
BF3·OEt2 was freshly distilled immediately prior to use. CCl4
(water Յ0.005%, over molecular sieves) was degassed by multiple
freeze-pump-thaw cycles. All other solvents and reagents were pur-
chased from commercial suppliers and used without further purifi-
cation. TLC experiments were carried out on aluminum sheets or
glass plates (Merck) coated with silica gel 60 F254; spots were visu-
alized with UV light (254 nm, 366 nm) and developed with phos-
phomolybdic acid reagent, which was prepared by dissolving phos-
phomolybdic acid (25 g), Ce(SO4)2·4H2O (10 g), and H2SO4
(60 mL) in water (940 mL).[17] Flash chromatography was carried
out on unmodified silica gel (25–40 μm, 60 Å, Macherey–Nagel, or
32–63 μm, 60 Å, Acros Organics). Melting points were determined
formed equally well into the carboxylic acid (S)-2. Phen-
guignardic acid (2) exhibited optical rotations of [α]D22
+227 (c = 0.57, CH3CN), +194 (c = 0.24, CDCl3), +200 (c
= 1.0, CH2Cl2) in the case of the S enantiomer and [α]D22
–197 (c = 0.25, CDCl3), –224 (c = 0.55, CH3CN) for its
optical antipode, whereas the optical rotation of natural 2
obtained by fermentation amounted to [α]2D3 = +116 (c =
0.18, CDCl3).[5]
=
=
Conclusions
(+)-Phenguignardic acid (2) and its optical antipode were
prepared in an ex-chiral-pool synthesis from commercially
available (R)-phenyllactic acid, which can alternatively be with a Krüss melting point apparatus and are uncorrected. NMR
prepared in a single step from d-phenylalanine.[16] The ab-
solute configuration of the quaternary stereocenter of natu-
ral 2 was determined to be S by comparison of the ECD
spectra were recorded with Bruker spectrometers (300, 400, or
600 MHz) and use of 5 mm probes and standard pulse sequences.
The spectra were measured in CDCl3, CD3CN, or CD2Cl2, and
the chemical shifts were referenced to the residual solvent signals
spectra and optical rotation values of the natural product
(CDCl3: δH = 7.26, δC = 77.16. CD2Cl2: δH = 5.32, δC = 53.84.
and the synthetic enantiomers. The presence of a benzyl
CD3CN δH = 1.94, δC = 118.26/1.32 ppm).[18] IR spectra were re-
(compound 2) rather than an alkyl substituent (compound
1) at the acetal center thus led to near perfectly inverted
ECD spectra, whereas exchange of the methyl group (com-
pound 9) for a carboxylic acid function (compound 2) had
corded with a Bruker Tensor 27 FTIR spectrometer and a diamond
ATR unit; mass spectra were measured with an Agilent/
Bruker XCT linear ion trap LC/MSD detector (ESI-MS). HRMS
(ESI) spectra were recorded with a Waters Q-TOF Ultima-III spec-
no significant effect.[11] In addition to a sensitivity against trometer with a dual source and a suitable external calibrant. LC/
MS solvents (Optima LC/MS®: water, methanol, and acetonitrile)
acid, which led to transacetalization and ring opening reac-
were purchased from Fisher Scientific (Germany). Analytical
tions, the benzylidene dioxolanones were susceptible to nu-
HPLC was carried out with a Knauer Smartline HPLC system
cleophilic attack, which might explain the low yields in the
(Berlin, Germany) in high pressure gradient mode fitted with two
last three steps and considerably limited the choice of suit-
pumps, a Knauer Smartline K-1050 (water) and a Knauer K-1001
able protecting groups.
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A. Stoye, D. Kowalczyk, T. Opatz
FULL PAPER
(acetonitrile or methanol), each fitted with a 10 mL pump head.
The system was connected to a K-2800 diode array detector. The
size of the injection loop was 20 μL; the total flow rate 1.0–
2.0 mLmin–1. An ACE3–C18 column (Advanced Chromatography
Technologies, 125ϫ4.6 mm, 3 μm particle size) or an ACE3–
C18PFP column (Advanced Chromatography Technologies,
150ϫ4.6 mm, 3 μm particle size) was used. The elution was per-
formed with a gradient of H2O and MeCN or MeOH at 25–40 °C
(acetonitrile) or 40–60 °C (methanol). A manually controlled col-
umn oven (Jet-Stream) was used for temperature adjustment during
analytical scale HPLC. Preparative NP- and RP-HPLC were car-
ried out with a Knauer Smartline HPLC system (high pressure gra-
dient) fitted with two K-1800 pumps (pump head size: each
100 mL), a diode array detector (S-2600), and a 2.0 mL injection
loop. When the HPLC was run in reversed-phase (RP) mode, the
elution was performed with an ACE5–C18 column (Advanced
Chromatography Technologies, 125ϫ21.2 mm, 5 μm particle size)
or an ACE5–C18PFP column (Advanced Chromatography Tech-
nologies, 150ϫ30 mm, 5 μm particle size) with a gradient of H2O
[pure, or with 0.1% (v/v) formic acid] and acetonitrile or methanol
at 21 °C and total flow rates of 26.2–52.4 mLmin–1 for columns of
21.2 mm diameter, and 37.5–56.3 mLmin–1 for 30 mm columns.
For normal-phase (NP) HPLC, a preparative column packed with
unmodified silica (125ϫ21.2 mm, 3 μm, NUCLEODUR®, Mach-
erey–Nagel) was used and elution was performed with a combina-
tion of n-hexane and propan-2-ol (isocratic mode) and a total flow
rate of 21.2 mLmin–1 at 21 °C. HPLC-MS was carried out with
an Agilent 1200 system (binary pump, column oven, auto sampler,
DAD) and either an Ascentis Express® C18 column (50ϫ2.1 mm,
core-shell, particle size: 2.7 μm) or an Ascentis Express® C8 column
(30ϫ2.1 mm, core-shell, particle size: 2.7 μm) with a gradient of
H2O containing formic acid (0.1%) for positive ionization or am-
monium hydrogen carbonate (15 mm) for negative ionization mode
and MeCN at a temperature of 50 °C and a total flow rate of 0.50–
1.0 mLmin–1. The mass spectra were recorded with use of positive
or negative electrospray ionization. Optical rotations were mea-
sured with a Perkin–Elmer 241 polarimeter at wavelengths of
546 nm and 578 nm (Hg lamp). The data were extrapolated to a
wavelength of λ = 589 nm by use of the Drude equation.[19] ECD
spectra were recorded with a Jasco J-815 CD spectrometer in a
quartz cuvette of pathlength 1.0 mm at 20.0 °C with a scanning
speed of 50 nmmin–1 (five repetitions) in the range of λ = 180–
350 nm. The samples for ECD measurements were prepared by dis-
solving the compounds in a suitable amount of acetonitrile to ob-
tain solutions in concentrations ranging from c = 0.3–1.5 mm.
Diastereomer 8a (trans): Rf = 0.58 [petroleum ether/ethyl acetate
5:1 (v/v)], m.p. 74.3–74.9 °C. 1H NMR, NOESY (400 MHz): δ =
7.35–7.19 (m, 10 H, aryl), 3.95 (dd, J = 5.3, 4.7 Hz, 1 H, 5-H), 3.06
(d, J = 14.1 Hz, 1 H, PhCH2A), 3.03 (dd, J = 14.6, 4.7 Hz, 1 H,
PhCH2ACHOR), 3.00 (d, J = 14.1 Hz, 1 H, PhCH2B), 2.93 (dd, J
= 14.6, 5.3 Hz, 1 H, PhCH2BCHOR), 1.26 (s, 3 H, CH3) ppm. 13C
NMR (75.5 MHz): δ = 172.6 (C-4), 135.7, 134.0 (C-1Ј,1ЈЈ) 131.0,
130.0, 128.6, 128.4 (C-2Ј,6Ј, C-2ЈЈ,6ЈЈ, C-3Ј,5Ј, C-3ЈЈ,5ЈЈ), 127.5,
127.1 (C-4Ј,C-4ЈЈ), 111.6 (C-2), 78.0 (C-5), 46.1 (PhCH2CHOR),
37.8 (PhCH ), 26.3 (CH ) ppm. FTIR (ATR): ν = 3066, 2894, 1781,
˜
2
3
1604, 1454, 1381, 1081, 907, 757, 720, 700, 635 cm–1. ESI-MS
(pos.): m/z
= 305.0 [M +
Na]+. HRMS (ESI): calcd. for
C18H18NaO3 [M + Na]+ 305.1154; found 305.1155.
Diastereomer 8b (cis): Rf = 0.50 [petroleum ether/ethyl acetate 5:1
(v/v)], 1H NMR, NOESY (400 MHz, CDCl3): δ = 7.42–7.23 (m,
10 H, aryl), 4.62 (dd, J = 8.2, 3.5 Hz, 1 H, 5-H), 3.05 (d, J =
14.2 Hz, 1 H, PhCH2A), 3.00 (dd,
J = 14.5, 3.5 Hz, 1 H
PhCH2ACHOR), 2.94 (d, J = 14.2 Hz, 1 H, PhCH2B), 2.50 (dd, J
= 14.5, 8.2 Hz, 1 H, PhCH2BCHOR), 1.54 (s, 3 H, CH3) ppm. 13C
NMR (75.5 MHz): δ = 172.3 (C-4), 136.4 (Cq), 134.3 (Cq), 130.9,
129.5, 128.5, 128.3 (C-2Ј,6Ј, C-2ЈЈ,6ЈЈ, C-3Ј,5Ј, C-3ЈЈ,5ЈЈ), 127.3,
127.1 (C-4Ј,4ЈЈ), 111.6 (C-2), 75.3 (C-5), 45.8 (PhCH2CHOR), 37.6
(PhCH ), 25.1 (CH ) ppm. FTIR (ATR): ν = 3064, 3031, 2927,
˜
2
3
1791, 1711, 1604, 1496, 1454, 1382, 966, 892, 755, 698 cm–1. ESI-
MS (pos.): m/z = 305.1 [M + Na]+. HRMS (ESI): calcd. for
C18H18NaO3 [M + Na]+ 305.1154; found 305.1146.
(2S,5Z)-2-Benzyl-5-benzylidene-2-methyl-1,3-dioxolan-4-one (ent-9)
Method A: A solution of 8a (104 mg, 370 μmol) in THF (3.00 mL)
was added dropwise at –78 °C to a stirred solution of lithium diiso-
propylamide (200 μL, 2 m, 400 μmol, 1.10 equiv.) in anhydrous
THF (2.00 mL). The temperature of the reaction mixture was in-
creased to –20 °C over a period of approximately 1 h, and the mix-
ture was then once more cooled to –78 °C. A solution of phenylse-
lenyl bromide (101 mg, 426 μmol, 1.15 equiv.) in THF (1.00 mL)
was then added dropwise. After stirring for 10 min at this tempera-
ture, the mixture was allowed to warm to 0 °C, and acetic acid
(1.43 mL) and aqueous hydrogen peroxide solution (30%, 570 μL,
5.58 mmol, 15.0 equiv.) were then added successively. The reaction
mixture was stirred for 2 h at 0 °C, after which the solution was
brought to slightly basic pH by addition of saturated aqueous so-
dium hydrogen carbonate solution. The mixture was extracted with
CH2Cl2 (3ϫ 40 mL), and the combined organic extracts were
washed with brine, dried with anhydrous magnesium sulfate, and
concentrated under reduced pressure. Flash chromatography on sil-
ica gel [petroleum ether/ethyl acetate 10:1 (v/v)] furnished ent-9
(35 mg, 34%) as a colorless oil. Rf = 0.56 [petroleum ether/ethyl
acetate 5:1 (v/v)].
(2S,5R)-2,5-Dibenzyl-2-methyl-1,3-dioxolan-4-one (8a) and (2R,5R)-
2,5-Dibenzyl-2-methyl-1,3-dioxolan-4-one (8b): A mixture of (R)-2-
hydroxy-3-phenylpropanoic acid (4.96 g, 29.8 mmol, 2.00 equiv.),
phenylpropan-2-one (2.00 g, 14.9 mmol), and BF3·OEt2 (48%,
15 mL) was stirred at room temperature. After the ketone had been
consumed (4 h), CH2Cl2 (20 mL) was added, and the reaction mix-
ture was carefully added to a saturated Na2CO3 solution (50 mL)
and stirred until the evolution of CO2 had ceased. After separation,
the aqueous phase was extracted with CH2Cl2 (50 mL). The com-
bined organic extracts were dried with anhydrous sodium sulfate
and filtered, and the solvents were evaporated. The residue (3.37 g)
was purified by flash chromatography on silica gel [petroleum
ether/ethyl acetate 10:1 (v/v)], affording the title compounds in 72%
combined yield (dr 1.5:1). The trans diastereomer 8a (1.74 g, 41%)
was obtained as colorless crystals, the cis diastereomer 8b (1.11 g,
27%) as a colorless oil. A third fraction contained both dia-
stereomers (188 mg, 4%).
Method B: A mixture of dioxolanone 8a (282 mg, 1.00 mmol) in
anhydrous, degassed CCl4 (3.00 mL), N-bromosuccinimide
(180 mg, 1.00 mmol, 1.00 equiv.), and AIBN (2.2 mg, 13 μmol,
1.3 mol-%) was heated at reflux for 3 h. After complete consump-
tion of the starting material, the reaction mixture was filtered
through a syringe tip filter (pore size: 0.20 μm). After washing of
the filter with CCl4 (5 mL), evaporation of the solvent, and purifi-
cation of the residue by flash chromatography on silica gel [petro-
leum ether/ethyl acetate 10:1 (v/v)], the 5-benzylidene-1,3-dioxolan-
4-one ent-9 (140 mg, 50%) was obtained as a colorless oil. Rf =
0.56 [petroleum ether/ethyl acetate 5:1 (v/v)]. [α]2D1 = –330 (c = 1.0,
CHCl3). 1H NMR (300 MHz): δ = 7.67–7.62 (m, 2 H, aryl-H),
7.44–7.30 (m, 3 H, aryl-H), 7.25 (mc, 5 H, aryl-H), 6.25 (s, 1 H,
PhCH=COR), 3.25 (d, J = 14.3 Hz, 1 H, PhCH2), 3.20 (d, J =
14.3 Hz, 1 H, PhCH2), 1.75 (s, 3 H, CH3) ppm. 13C NMR
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Synthesis of a Guignardia bidwellii Metabolite
(75.5 MHz, CDCl3): δ = 163.7 (C-4), 137.1 (C-5), 133.0, 132.8, (C-
1Ј,1ЈЈ) 130.9, 129.6 128.7, 128.5 (C-2Ј,6Ј, C-2ЈЈ,6ЈЈ, C-3Ј,5Ј, C-
chloride (5.01 mL, 7.12 g, 63.0 mmol, 1.5 equiv.), were added
slowly to a stirred solution (0 °C) of 1-hydroxy-3-phenylpropan-
3ЈЈ,5ЈЈ), 128.6 (C-4Ј or 4ЈЈ), 127.7 (C-4Ј or 4ЈЈ), 112.0 (C-2), 107.8 2-one (10, 6.30 g, 42.0 mmol) in dichloromethane (100 mL). The
(PhCH=CHOR), 46.0 (PhCH ), 25.5 (CH ) ppm. FTIR (ATR): ν cooling bath was removed, and the mixture was stirred for 24 h
˜
2
3
= 3063, 2937, 1775, 1711, 1604, 1496, 1454, 1380, 1334, 1091, 710,
678 cm–1. ESI-MS (pos.): m/z = 281.0 [M + H]+, 303.0 [M +
Na]+. HRMS (ESI, pos.): calcd. for C18H16NaO3 [M + Na]+
303.0997; found 303.0992.
and then mixed with saturated NaHCO3 solution (100 mL). After
phase separation, extraction with dichloromethane (2ϫ 75 mL),
and drying of the combined organic layers over anhydrous Na2SO4,
the solvent was evaporated under reduced pressure. The remaining
residue (11.5 g) was purified by silica gel flash chromatography [pe-
troleum ether/ethyl acetate 6:1 (v/v)] to yield the title compound
(7.01 g, 30.9 mmol, 74%) as a slightly yellow solid. Rf = 0.50 [petro-
(2R,5Z)-2-Benzyl-5-benzylidene-2-methyl-1,3-dioxolan-4-one
(9):
The title compound 9 was obtained by the same methodology as
described for the synthesis of ent-9 (Method A): compound 9
(32.0 mg, 29%) was synthesized from 8b (110 mg, 0.39 mmol) and
isolated as a colorless oil. Rf = 0.56 [petroleum ether/ethyl acetate
1
leum ether/ethyl acetate 3:1 (v/v)]. H NMR (300 MHz): δ = 7.34
(m, 5 H, aryl), 4.80 (s, 2 H, 1-H), 4.17 (s, 2 H, 3-H), 3.75 (s, 2 H,
O=CCH2Cl) ppm. 13C NMR (75.5 MHz): δ = 200.1 (O=CCH2Cl),
166.8 (C-2), 132.5 (C-1Ј), 129.5 (C-2Ј,6Ј), 129.0 (C-3Ј,5Ј) 127.6 (C-
4Ј), 68.5 (C-1), 46.4 (C-3), 40.4 (O=CCH2Cl) ppm. FTIR (ATR):
5:1 (v/v)]. [α]2D1 = +360 (c = 1.0, CHCl ). FTIR (ATR): ν = 3063,
˜
3
2927, 1780, 1669, 1603, 1495, 1451, 1364, 1262, 1074, 710,
678 cm–1. ESI-MS (pos.): m/z = 281.1 [M + H]+, 303.1 [M +
Na]+. HRMS (ESI): calcd. for C18H16NaO3 [M + Na]+ 303.0997;
found 303.0993. All other spectroscopic data were identical to the
data given for compound ent-9 (vide supra).
ν = 3034, 1764, 1722, 1418, 1327, 1183, 1169, 895, 789, 697 cm–1.
˜
ESI-MS (pos.): m/z = 226.9 [M + H]+. HRMS (ESI): calcd. for
C11H11ClNaO3 [M + Na]+ 249.0294; found 249.0297.
1-Hydroxy-3-phenylpropan-2-one (10): The title compound was pre-
pared by a protocol by Waters et al.[12b] 3-Phenylprop-2-yn-1-ol
(15.8 g, 119 mmol), aqueous KOH (30 wt.-%, 2.26 g, 121 mmol),
and n-propanethiol (11.2 mL, 124 mmol) were dissolved in aceto-
nitrile (150 mL) under inert atmosphere and stirred at 60 °C. After
6 h, about half of the starting material had been consumed (TLC,
hexanes/ethyl acetate 5:2 v/v, starting material: Rf = 0.40). After
addition of a further portion of propanethiol (11.2 mL, 124 mmol)
and KOH (30 wt.-%, 2.26 g, 121 mmol), the reaction mixture was
stirred for another 12 h. After cooling to room temp. and removal
of the solvent under reduced pressure, the yellow oil was dissolved
in tert-butyl methyl ether (TBME) and filtered through a pad of
silica. The silica was washed with TBME (300 mL), and evapora-
tion of the solvent under reduced pressure furnished 3-phenyl-2-
(propylthio)prop-2-en-1-ol as a pale yellow oil (23.0 g, 111 mmol,
93%). Rf = 0.47 (petroleum ether/ethyl acetate 5:2). Sulfuric acid
(1 m, 120 mL) was added to a stirred suspension of 3-phenyl-2-
(propylthio)prop-2-en-1-ol (22.6 g, 108 mmol) in ethanol (120 mL)
in a three-necked round-bottomed flask fitted with a nitrogen inlet
and a gas outlet connected to a gas wash bottle containing aqueous
H2O2 (30%). The resulting yellow solution was heated to 65 °C for
8 h under a moderate stream of nitrogen to remove the n-prop-
anethiol formed. After the system had cooled to room temp., water
(50 mL) was added, and the resulting solution was washed with
petroleum ether (250 mL). The organic layer was discarded. Brine
(120 mL) was added to the aqueous phase, and the product was
extracted with ethyl acetate (3ϫ 150 mL). The combined organic
extracts were dried with Na2SO4, filtered, and concentrated under
reduced pressure (T Յ 25 °C). The residue was dissolved in ethyl
acetate (35 mL) and filtered through a pad of celite. After the fil-
trate had been cooled to –5 °C, n-heptane (150 mL) was added
slowly (during ca. 20 min), and the product began to precipitate.
The flask was placed in the refrigerator (–24 °C) for at least 1 h,
and the title compound 10 was obtained as colorless crystals by
filtration (12.5 g, 83.2 mmol, 77%). Rf = 0.28 [petroleum ether/
ethyl acetate 5:2 (v/v)], m.p. 49.2–49.9 °C, ref.[12] 45–46 °C. 1H
NMR (400 MHz): δ = 7.38–7.20 (m, 5 H, aryl-H), 4.29 (s, 2 H, 1-
H), 3.72 (s, 2 H, 3-H) ppm. 13C NMR (100.6 MHz): δ = 207.5 (C-
2), 132.8 (C-1Ј), 129.4 (C-2Ј,6Ј), 128.9 (C-3Ј,5Ј) 127.5 (C-4Ј), 67.7
(2S,4R)-[2,4-Dibenzyl-5-oxo-1,3-dioxolan-2-yl]methyl Chloroacetate
(12a) and (2R,4R)-[2,4-Dibenzyl-5-oxo-1,3-dioxolan-2-yl]methyl
Chloroacetate (12b): A mixture of (R)-2-hydroxy-3-phenylprop-
anoic acid (4.96 g, 29.8 mmol, 2.0 equiv.), ketone 11 (2.00 g,
14.9 mmol), and BF3·OEt2 (48%, 15 mL) was stirred at room tem-
perature for 24 h. After the ketone had been consumed (4 h),
CH2Cl2 (20 mL) was added, and the reaction mixture was added
carefully to a saturated Na2CO3 solution (50 mL) and stirred until
the evolution of CO2 had ceased. After separation, the aqueous
phase was extracted with CH2Cl2 (50 mL). The combined organic
extracts were dried with anhydrous sodium sulfate and filtered, and
the solvents were evaporated. Purification of the residue (3.37 g) by
fast flash chromatography (short column) on silica gel [petroleum
ether/ethyl acetate 10:1 (v/v)] afforded the title compounds (5.27 g,
94% combined yield) as diastereomeric mixture (dr 2:1). Separation
of the diastereomers was achieved by flash chromatography (long
column) on silica gel with petroleum ether/diethyl ether 5:1 (v/v);
the cis diastereomer 12a (1.79 g, 32%) was obtained as colorless
crystals (Rf = 0.10) and the trans diastereomer 12b (1.04 g, 27%)
as a slightly yellowish oil (Rf = 0.14). A third fraction contained
both diastereomers (0.51 g, 9%).
Diastereomer 12a: Rf = 0.55 [petroleum ether/ethyl acetate 5:2
(v/v)], m.p. 76.8–77.9 °C. [α]2D1 = +27.5 (c = 1.0, CHCl3). 1H NMR,
COSY, NOESY (400 MHz, CDCl3): δ = 7.37–7.20 (m, 10 H, aryl-
H), 4.75 (dd, J = 7.8, 3.8 Hz, 1 H, 4-H), 4.31 (d, J = 11.9 Hz, 1 H,
CH2AO2CCH2Cl), 4.20 (d, J = 11.9 Hz, 1 H, CH2BO2CCH2Cl),
4.06 (s, 2 H, O2CCH2Cl), 2.98 (d, J = 14.6 Hz, 1 H, PhCH2A), 2.93
(dd, J = 14.6, 3.8 Hz, 1 H, PhCH2ACHOR), 2.83 (d, J = 14.6 Hz,
B
1 H, PhCH2
)
2.43 (dd,
J
=
14.6, 7.8 Hz, 1 H,
PhCH2BCHOR) ppm. 13C NMR, HSQC, HMBC (100.6 MHz): δ
= 171.7 (C=O), 163.3 (C-5), 135.8 (C-1Ј), 132.4 (C-1ЈЈ), 130.8 (C-
2Ј,6Ј), 129.5 (C-2ЈЈ,6ЈЈ), 128.6, 128.5 (C-3Ј,5Ј,3ЈЈ,5ЈЈ), 127.7, 127.2,
(C-4Ј,4ЈЈ) 108.5 (C-2), 76.7 (C-4), 68.0 (CH2O2CCH2Cl), 47.1
(PhCH2), 40.4 (O=CCH2Cl), 37.8 (PhCH2CHOR) ppm. FTIR
(ATR): ν = 3032, 1799, 1749, 1283, 1227, 1165, 1088, 958,
˜
699 cm–1. ESI-MS (pos.): m/z = 397.1 [M + Na]+. HRMS (ESI):
calcd. for C20H19ClNaO5 [M + Na]+ 397.0819; found 397.0821.
Diastereomer 12b: Rf = 0.59 [petroleum ether/ethyl acetate 5:2
(v/v)]. [α]2D1 = +33.3 (c = 1.0, CHCl3). H NMR, COSY, NOESY
1
(C-1), 45.8 (C-3) ppm. FTIR (ATR): ν = 3423, 3063, 3030, 2908,
˜
1719, 1603, 1495, 1454, 1328, 1250, 1003, 794, 755, 699, 679 cm–1.
The analytical data match those reported in the literature.[12]
(400 MHz, CDCl3): δ = 7.33–7.13 (m, 10 H, aryl-H), 4.08 (d, J =
11.9 Hz, 1 H, CH2AO2CCH2Cl), 4.06 (s, 2 H, O2CCH2Cl), 3.86 (dd,
2-Oxo-3-phenylpropyl Chloroacetate (11): N,N-Diisopropylethyl-
amine (10.9 mL, 8.09 g, 63.0 mmol, 1.5 equiv.) and chloroacetyl
J
=
3.8, 6.4 Hz, 1 H, 4-H), 3.85 (d,
J = 11.9 Hz, 1 H,
CH2BO2CCH2Cl), 3.12 (d, J = 14.1 Hz, 1 H, PhCH2A), 3.02 (d, J
Eur. J. Org. Chem. 2013, 5952–5960
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
5957

A. Stoye, D. Kowalczyk, T. Opatz
FULL PAPER
= 14.1 Hz, 1 H, PhCH2B), 3.02 (dd, J = 14.5, 3.8 Hz, 1 H,
ture and stirred for 15 h {TLC: Reactant: Rf = 0.54, product: Rf
= 0.35 [petroleum ether/ethyl acetate 5:3 (v/v)]}. The solvent was
removed in vacuo at the lowest possible temperature (T Յ 40 °C).
PhCH2ACHOR) 2.93 (dd,
J
=
14.5, 6.4 Hz, 1 H,
PhCH2BCHOR) ppm. 13C NMR, HSQC, HMBC (100.6 MHz,
CDCl3): δ = 171.4 (C=O), 166.4 (C-4), 135.2 (C-1Ј), 132.1 (C-1ЈЈ), Toluene (ca. 30 mL) was added and, after evaporation of the sol-
131.0 (C-2Ј,6Ј), 129.6 (C-2ЈЈ,6ЈЈ), 128.7, 128.5 (C-3Ј,5Ј, C-3ЈЈ,5ЈЈ), vent under reduced pressure, the remaining residue was dissolved in
127.8, 127.3 (C-4Ј, C-4ЈЈ), 108.8 (C-2), 75.6 (C-4), 67.3 dichloromethane (10.0 mL) and cooled to 0 °C. TESOTf (660 μL,
(CH2O2CCH2Cl), 41.8 (PhCH2), 40.4 (O=CCH2Cl), 37.6 770 mg, 2.90 mmol, 1.20 equiv.) and NEt3 (610 μL, 440 mg,
(PhCH CHOR) ppm. FTIR (ATR): ν = 3032, 1800, 1766, 1285, 4.36 mmol, 1.80 equiv.) were then added dropwise to the stirred
˜
2
1090, 957, 756, 701 cm–1. ESI-MS (pos.): m/z = 397.1 [M + Na]+.
solution of the crude alcohol 13b in dichloromethane (10 mL) at
HRMS (ESI): calcd. for C20H19ClNaO5 [M + Na]+ 397.0819; 0 °C. Stirring was continued at room temp. until TLC showed com-
found 397.0808.
plete conversion (2 h). Sometimes the addition of a second portion
of TESOTf (330 μL, 1.45 mmol, 0.60 equiv.)/triethylamine (305 μL,
2.18 mmol, 0.90 equiv.), followed by stirring for 15 h at room temp.,
was required for the reaction to run to completion. The mixture
was then cooled to 0 °C and quenched with saturated aqueous
NH4Cl solution (7 mL). After phase separation, extraction with
CH2Cl2 (2ϫ 20 mL), and washing of the combined organic extracts
with brine (75 mL), the organic phase was dried with anhydrous
Na2SO4. Evaporation of the solvent and purification by preparative
RP-HPLC on an ACE5–C18PFP column, (150 ϫ30 mm, 5 μm)
with a mixture of MeCN/H2O 77:23 (v/v) as eluent (total flow rate:
37.5 mLmin–1), afforded the TES-protected dioxolanones: the de-
sired 14b (296 mg, 30%, two steps from 12b, Rt = 12.3 min), epi-
merized 14a (118 mg, 12%, Rt = 11.0 min), and a diastereomeric
mixture of 14a/14b (28.0 mg, 3%), as well as 14c (36.0 mg, 9%,
Rt = 9.3 min), an open-chain and triethylsilyl-protected byproduct
(Scheme 4), were obtained as colorless oils.
(2S,5R)-2,5-Dibenzyl-2-{[(triethylsilyl)oxy]methyl}-1,3-dioxolan-4-
one (14a) – Cleavage with HDTC: An HDTC solution [generated
in situ (see ref.[13]), 35.6 mL, 0.375 molL–1, 13.4 mmol, 3.00 equiv.]
was added at room temperature to a stirred solution of 12a (1.67 g,
4.46 mmol) in a mixture of 2,6-lutidine (32.1 mL) and acetic acid
(10.8 mL). The reaction mixture was stirred at this temperature un-
til the starting material had been consumed (20 min) {TLC: reac-
tant: Rf = 0.54, product: Rf = 0.29 [petroleum ether/ethyl acetate
5:3 (v/v)]}, toluene (100 mL) was added, and the solvents were
evaporated in vacuo at the lowest possible temperature (T Յ 30 °C)
to afford the corresponding dioxolanone 13a, bearing a free 2-
hydroxymethyl group, as a crude product. It was used immediately,
without further purification, in the next step for best results. TE-
SOTf (1.22 mL, 5.35 mmol, 1.20 equiv.) and NEt3 (1.13 mL,
8.03 mmol, 1.80 equiv.) were added dropwise at 0 °C to a stirred
solution of the crude alcohol 13a in dichloromethane (30 mL). Stir-
ring was continued at room temp., until TLC showed complete con-
version (ca. 2 h). Sometimes addition of a second portion of TE-
SOTf (610 μL, 2.68 mmol, 0.60 equiv.)/triethylamine (560 μL,
4.02 mmol, 0.90 equiv.), followed by stirring for 15 h at room temp.,
was needed for the reaction to run to completion. The mixture was
then cooled to 0 °C and quenched with saturated aqueous NH4Cl
solution (10 mL). After phase separation, extraction with CH2Cl2
(2ϫ 30 mL), and washing of the combined organic extracts with
brine (75 mL), the organic phase was dried with sodium sulfate.
Evaporation of the solvent and purification by preparative RP-
HPLC [ACE5–C1 8 , H2 O/MeCN 23:77 (v/v), total flow:
26.2 mL min–1], afforded the TES-protected dioxolanone 14a
(860 mg, 39% over two steps from 12a) as a colorless oil.
Compound 14b: Rt = 12.3 min. Rf = 0.79 [petroleum ether/ethyl
acetate 5:1 (v/v)]. [α]2D1 = +36.4 (c = 1.0, CH3CN). 1H NMR, COSY,
NOESY (400 MHz, CD2Cl2): δ = 7.32–7.24 (m, 8 H, aryl-H), 7.18–
7.16 (m, 2 H, aryl-H), 3.94 (dd, J = 6.2, 5.1 Hz, 1 H, 5-H), 3.53 (d,
J = 11.2 Hz, 1 H, CH2AOTES), 3.26 (d, J = 11.2 Hz, 1 H,
CH2BOTES), 3.13 (d, J = 14.1 Hz, 1 H, PhCH2A), 3.01 (d, J =
14.1 Hz, 1 H, PhCH2B), 2.98–2.96 (m, 2 H, PhCH2CHOR), 0.96 (t,
J = 7.9 Hz, 9 H, SiCH2 CH3 ), 0.60 (q, J = 7.9 Hz, 6 H,
SiCH2CH3) ppm. 13C NMR, HSQC, HMBC (100.6 MHz,
CD2Cl2): δ = 172.1 (C-4), 136.2 (C-1ЈЈ), 133.7 (C-1Ј), 131.0 (C-
2ЈЈ,6ЈЈ), 129.6 (C-2Ј,6Ј), 128.27/128.28 (C-3Ј,5Ј,3ЈЈ,5ЈЈ), 127.1/126.9
(C-4Ј,4ЈЈ), 111.4 (C-2), 75.6 (C-5), 66.5 (CH2OSiEt3), 41.0 (PhCH2)
37.8 (PhCH2CHOR), 6.4 (SiCH2CH3), 4.2 (SiCH2CH3) ppm.
FTIR (ATR): ν = 3032, 2955, 2876, 1799, 1497, 1455, 1239, 1173,
˜
1080, 802, 745, 698 cm–1. ESI-MS (pos.): m/z = 435.2 [M + Na]+.
HRMS (ESI): calcd. for C24H32NaO4Si [M + Na]+ 435.1968; found
435.1964.
Compound 14a: Rt = 11.6 min Rf = 0.79 [petroleum ether/ethyl acet-
ate 5:1 (v/v)], [α]2D1 = +35.7 (c = 1.0, CH3CN). H NMR, COSY,
1
NOESY (400 MHz, CD2Cl2): δ = 7.37–7.16 (m, 10 H, aryl-H), 4.70
(dd, J = 9.0, 3.6 Hz, 1 H, 5-H), 3.75 (d, J = 11.1 Hz, 1 H, CH2
A-
(2S,5Z)-2-Benzyl-5-benzylidene-2-(hydroxymethyl)-1,3-dioxolan-4-
one [(S)-15] – Pathway A: A mixture of dioxolanone 14a (382 mg,
926 μmol) in anhydrous, degassed CCl4 (2.78 mL), N-bromosuc-
cinimide (167 mg, 926 μmol, 1.00 equiv.), and AIBN (2.03 mg,
11.4 μmol, 1.3 mol-%) was heated at reflux. After complete disap-
pearance of the starting material (1 h), the reaction mixture was
filtered through a syringe tip filter (pore size: 0.20 μm), and the
filter was then washed with CCl4 (ca. 10 mL). Evaporation of the
solvent under reduced pressure and purification by preparative NP-
HPLC [n-hexane/propan-2-ol 95:5 (v/v)] afforded (S)-15 (100 mg,
338 μmol, 36%) as a colorless oil. Rt = 28 min (column: unmodi-
OSiEt3), 3.71 (d, J = 11.1 Hz, 1 H, CH2BOSiEt3.), 2.98 (d, J =
14.3 Hz, 1 H, PhCH2A ), 2.87 (dd, J = 14.3, 3.6 Hz, 1 H,
PhCH2ACHOR), 2.85 (d, J = 14.3 Hz, 1 H, PhCH2B), 2.43 (dd,
J = 14.3, 9.0 Hz, 1 H, PhCH2BCHOR), 0.92 (t, J = 7.9 Hz, 9 H,
CH2CH3), 0.58 (q, J = 7.9 Hz, 6 H, CH2CH3) ppm. 13C NMR,
HSQC, HMBC (100.6 MHz, CD2Cl2): δ = 173.0 (C-4), 137.1 (C-
1ЈЈ), 134.0 (C-1Ј), 131.3 (C-2ЈЈ,6ЈЈ), 129.7 (C-2Ј,6Ј), 128.7, 128.5 (C-
3Ј,5Ј, C-3ЈЈ,5ЈЈ), 127.6, 127.2 (C-4Ј, C-4ЈЈ), 111.0 (C-2), 77.6 (C-5),
67.6 (CH2OSiEt3), 41.0 (PhCH2), 38.6 (PhCH2CHOR), 6.7
(SiCH CH ), 4.5 (SiCH CH ) ppm. FTIR (ATR): ν = 3032, 2955,
˜
2
3
2
3
2876, 1797, 1497, 1455, 1236, 1131, 896, 879 796, 744, 729,
fied silica, 125ϫ21.2 mm, total flow rate: 21.2 mLmin–1). [α]2D1
=
–
1
6 9 8 c m
. E S I - M S ( p o s . ) : m / z = 4 3 5 . 2
+320 (c = 0.5, CH3CN). 1H NMR, COSY, NOESY (600 MHz,
CD3CN): δ = 7.73 (mc, 2 H, 2ЈЈ,6ЈЈ-H), 7.44 (mc, 2 H, 3ЈЈ,5ЈЈ-H),
7.37 (mc, 1 H, 4ЈЈ-H), 7.32–7.29 (m, 2 H, 2Ј,6Ј-H), 7.26–7.24 (m,
[M + Na]+. HRMS (ESI): calcd. for C24H32NaO4Si [M + Na]+
435.1968; found 435.1964.
(2R,5R)-2,5-Dibenzyl-2-{[(triethylsilyl)oxy]methyl}-1,3-dioxolan-4- 3 H, 3Ј,4Ј,5Ј-H), 6.15 (s, 1 H, PhCH=COR), 3.85 (dd, J = 12.8,
one (14b) – Deprotection Procedure with Thiourea: A stirred solu-
tion of 12b (906 mg, 2.42 mmol) in DMF (5.00 mL) was treated
with thiourea (441 mg, 5.81 mmol, 2.40 equiv.) at room tempera-
6.2 Hz, 1 H, CH2OH), 3.83 (dd, J = 12.8, 6.2 Hz, 1 H CH2OH),
3.57 (t, J = 6.2 Hz, 1 H, OH), 3.30 (d, J = 14.3 Hz, 1 H PhCH2A),
3.30 (d, J = 14.3 Hz, 1 H PhCH2B) ppm. 13C NMR, HSQC,
5958
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© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2013, 5952–5960

Synthesis of a Guignardia bidwellii Metabolite
HMBC (151 MHz, CD3CN): δ = 163.6 (C-4), 136.6 (C-5), 133.1 was filtered through a syringe tip filter (0.2 μm, PTFE) and purified
(C-1ЈЈ), 132.6 (C-1Ј), 131.0 (C-2Ј,6Ј), 129.5 (C-2ЈЈ,6ЈЈ), 128.7 (C-
by preparative HPLC [ACE5–C18PFP, H2O (+0.1% formic acid)/
3ЈЈ,5ЈЈ), 128.4 (C-4ЈЈ), 128.2 (C-3Ј,5Ј), 127.3 (C-4Ј), 111.6 acetonitrile 40:60Ǟ0:100 (v/v) over 20 min, flow rate:
(PhCH=COR) 105.5 (C-2), 64.4 (CH2OH) 39.7 (PhCH2) ppm. 37.5 mLmin–1, Rt = 7.0 min.] to furnish (S)-2 (15.4 mg, 45%) from
FTIR (ATR): ν = 3435, 3064, 2928, 1783, 1453, 1195, 1183, 947,
(S)-16, or (S)-2 (42.0 mg, 45%) from (S)-15, as a colorless oil in
˜
786, 675 cm–1. ESI-MS (pos.): m/z = 319.1 [M + Na]+. HRMS each case.
(ESI, pos.): calcd. for C18H17O4 [M + H]+ 297.1127; found
297.1136.
Compound (R)-2 (18.2 mg, 35%) was synthesized in the same man-
ner from (R)-15 (49.9 mg, 168 μmol).
(2S,5Z)-2-Benzyl-5-benzylidene-2-{[(triethylsilyl)oxy]methyl}-1,3-di-
oxolan-4-one [(S)-16] – Pathway B: A mixture of dioxolanone 14a
(236 mg, 573 μmol) in anhydrous, degassed CCl4 (2.00 mL), N-
bromosuccinimide (103 mg, 573 μmol, 1.00 equiv.), 2,6-di-tert-but-
ylpyridine (133 μL, 1.1 equiv.), and AIBN (1.22 mg, 7.43 μmol,
1.3 mol-%) was heated to reflux (bath temperature 85–90 °C). The
reaction was monitored every 2 h by analytical HPLC [ACE3–C18
H2 O/MeOH 30:70Ǟ20:80 (v/v) over 20 min, flow rate:
1.50 mLmin–1 Rt = 25 min, 60 °C]. After complete disappearance
of the starting material (8 h), the reaction mixture was filtered
through a syringe tip filter (pore size: 0.20 μm), and the filter was
then washed with CCl4 (10 mL). Evaporation of the solvent under
reduced pressure at the lowest possible temperature (T Յ 40 °C),
after purification by preparative RP-HPLC [H2O/MeOH
30:70Ǟ20:80 (v/v) over 25 min: ACE5–C18, total flow rate:
26.2 mLmin–1] gave the title compound as a colorless oil (54.5 mg,
133 μmol, 23%, Rt = 28 min), still containing several impurities. It
was used in the oxidation step without further characterization.
Compound (S)-2: [α]2D2 = +227 (c = 0.57, CH3CN),[20] +194 (c =
0.24, CDCl3), +200 (c = 1.0, CH2Cl2). Compound (R)-2: [α]2D2
=
–197 (c = 0.25, CDCl3), –224 (c = 0.55, CH3CN). Compounds 2:
1H NMR (400 MHz): δ = 7.69 (mc, 2 H, 2Ј,6Ј-H), 7.43 (mc, 2 H,
3Ј,5Ј-H), 7.37 (mc, 1 H, 4Ј-H), 7.34–7.29 (m, 2 H, 2ЈЈ,6ЈЈ-H), 7.27–
7.22 (m, 3 H, 3ЈЈ,4ЈЈ,5ЈЈ-H), 6.25 (s, 1 H, PhCH=COR), 3.60 (d, J
= 14.6 Hz, 1 H, PhCH2A), 3.54 (d, J = 14.6 Hz, 1 H PhCH2B) ppm.
13C NMR (100.6 MHz, CD3CN): δ = 166.8 (CO2H), 163.3 (C-5),
136.6 (C-4), 133.1 (C-1Ј), 132.4 (C-1ЈЈ), 132.0 (C-2ЈЈ,6ЈЈ), 130.6 (C-
2Ј,6Ј), 130.1 (C-4Ј), 129.8 (C-3Ј,5Ј), 129.2 (C-3ЈЈ,5ЈЈ), 128.6 (C-4ЈЈ),
109.3 (C-2), 106.3 (PhCH=COR), 40.8 (PhCH2) ppm. FTIR
(ATR): ν = 3460, 3060, 1800, 1743, 1359, 1299, 1268, 1193, 1181,
˜
935, 866 cm–1. ESI-MS (neg.): m/z = 309.2 [M – H]–. HRMS (ESI,
pos.): calcd. for C18H14O5Na [M + Na]+ 333.0739; found 333.0753
[(S)-2]; found 333.0746 [(R)-2].
Supporting Information (see footnote on the first page of this arti-
cle): Copies of NMR spectra for all new compounds.
(2R,5Z)-2-Benzyl-5-benzylidene-2-(hydroxymethyl)-1,3-dioxolan-4-
one [(R)-15]/(2R,5Z)-2-Benzyl-5-benzylidene-2-{[(triethylsilyl)oxy]-
methyl}-1,3-dioxolan-4-one [(R)-16] – Pathway B: A mixture of di-
oxolanone 14b (238 mg, 577 μmol) in anhydrous, degassed CCl4
(2.00 mL), N-bromosuccinimide (104 mg, 577 μmol, 1.00 equiv.),
2,6-di-tert-butylpyridine (142 μL, 121 mg, 635 μmol, 1.10 equiv.),
and AIBN (1.22 mg, 7.44 μmol, 1.3 mol-%) was heated to reflux.
The reaction was monitored by analytical RP-HPLC [ACE3–
Acknowledgment
The authors thank the Rhineland-Palatinate center for Integrated
Natural Products Research for financial support. The help of Dr.
N. Hanold (Mainz) and Dr. J. C. Liermann (Mainz) for mass spec-
trometry and NMR spectroscopy is gartefully acknowledged. An
authentic sample of natural 2 was kindly provided by Prof. E.
Thines and I. Buckel (IBWF, Kaiserslautern). We also thank Prof.
H. Frey for granting accessto his ECD spectrometer.
C
18PFP, H2O/MeOH 30:70Ǟ20:80 (v/v) over 20 min, flow rate:
1.25 mLmin–1, Rt = 26 min, 60 °C]. After the reaction mixture had
been heated at reflux for 90 min, NBS (104 mg, 577 μmol,
1.00 equiv.) and AIBN (1.22 mg, 7.44 μmol, 1.3 mol-%) were
added, and after 4 h total reaction time the mixture was filtered
through a syringe tip filter (pore size: 0.20 μm), and the filter was
then washed with CCl4 (10 mL). Evaporation of the solvent under
reduced pressure at the lowest possible temperature (T Յ 40 °C),
after purification by preparative RP-HPLC [H2O/MeOH 30:70 v/v
(0 min)Ǟ20:80 v/v (25 min, flow rate 38.0 mL min–1, ACE5–
C18PFP], afforded (R)-15 (56.7 mg, 190 μmol, 33%, Rt = 7 min)
and (R)-16 (4.0 mg, 9.7 μmol, 2%, Rt = 37 min), both as colorless
oils. Compound (R)-15. [α]2D1 = –323 (c = 0.42, CH3CN).
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All other analytical data for compound (R)-15 were identical to
those for (S)-15.
(2S,4Z)-2-Benzyl-4-benzylidene-5-oxo-1,3-dioxolane-2-carboxylic
Acid [(S)-Phenguignardic Acid (S)-2, from (S)-15/or (S)-16] and (R)-
Phenguignardic Acid [(R)-2, from (R)-15] – General Procedure for
the Oxidation of the Primary Alcohols (S)-15 and (R)-15 as well
as the Alkoxysilane (S)-16, to the Corresponding Carboxylic Acids:
PhI(OAc)2 [489 mg in the case of (S)-15, 177 mg in that of (S)-16,
5 equiv.] and TEMPO [23.8 mg in the case of (S)-15, 8.6 mg in that
of (S)-16, 0.5 equiv.] were added at room temp. to a stirred solution
either of the alcohol (S)-15 (90.0 mg, 304 μmol) or of the TES-
protected alcohol (S)-16 (45.0 mg, 110 μmol) in a mixture of aceto-
nitrile/water 1:1 (v/v) [10.2 mL for (S)-15, 3.40 mL for (S)-16], and
the reaction mixture was stirred either for 24 h (alcohols) or for 3
days in the case of the alkoxysilane (S)-16. The reaction mixture
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Eur. J. Org. Chem. 2013, 5952–5960
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
5959

A. Stoye, D. Kowalczyk, T. Opatz
FULL PAPER
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[20] Although the specific optical rotation of phenguignardic acid
isolated from Guignardia bidwellii was lower than that of syn-
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in carboxylic acids can originate from variations in the water
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the optical purity by chiral HPLC failed.
Received: April 11, 2013
Published Online: July 24, 2013
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© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2013, 5952–5960